The release of environmental contaminants from underground storage tanks and associated devices such as piping, fittings, flanges, valves, and pumps by leakage, spills, overfilling, etc. is a serious, pervasive source of environmental pollution and a problem which has been taken up by numerous regulatory agencies.
In the United States, for example, a number of rules and regulations addressing this problem have been promulgated by the Environmental Protection Agency (EPA). Among other things, the EPA standards require that regulated tanks and piping be monitored for the leakage or other release of hazardous substances. This regulation can be complied with by employing a leak detection and monitoring system having the following minimum capabilities for tightness testing of tanks and piping:
Leak Detection Rate 0.10 gallon per hour PA1 Probability of Detection .gtoreq.95 percent PA1 Probability of a False Alarm .ltoreq.5 percent
Aside from the environmental pollution that results, leaks from containment vessels are undesirable because of the safety hazards and consequent liability the leaked substance may present and/or because of economic factors. For example, as the cost of crude oil increases, the economic penalties appurtenant to leaks of the oil and products refined from it become increasingly severe. Such considerations have led the American Petroleum Institute to recommend a leak detection threshold rate of 0.05 gallons per hours.
A comparable problem exists in aboveground storage systems. Leaks from aboveground tanks, pipes, and other containment vessels cause significant contamination of soil, air, surface water, etc. in the surrounding environment. And, again, no satisfactory system for monitoring such systems and detecting leaks with a satisfactory level of accuracy has been found to currently be available.
It is nowadays a widespread if not universal practice--at least in new construction--to store fluids and materials with a fluid phase in a system which has a primary containment for the stored material and a secondary containment enveloping or jacketing the primary containment. The secondary containment keeps stored materials from reaching the surrounding environs if there is a leak in the primary containment or such a leak develops. Also, the secondary containment keeps fluid in the surrounding environs from passing through such a leak and contaminating material stored in the primary containment.
For the same reasons, it is currently a common practice to jacket other fluid handling devices--pipes, valves, flanges, pumps, etc.
U.S. Pat. No.: 4,676,093 issued 30 Jun. 1987 to Pugnale et al. for DOUBLE-WALL UNDERGROUND TANK; U.S. Pat. No. 4,561,292 issued 31 Dec. 1985 to Pugnale et al. for DOUBLE-WALLED UNDERGROUND CONTAINER FOR STORING LIQUIDS AND LEAK DETECTING MEANS THEREFORE; and U.S. Pat. No. 4,936,705 issued 26 Jun. 1990 to Schneider for RESERVOIR for UNDERGROUND TANKS disclose leak detection systems for storage units of the character described in the preceding paragraph. In these patented systems, the space between primary and secondary containments is filled with a leak detecting liquid; and a drop in the level of this liquid is taken as evidence that a leak has occurred in the primary containment or in the secondary containment or that leaks have developed in both containments.
The only leak detecting fluid identified in the preceding patents is one "anti-freeze solution" of unspecified character and therefore presumably a conventional water-based one.
The use of antifreeze solutions and other aqueous leak detecting liquids is undesirable because of the corrosion they may cause by metal solubilization and/or by enabling electrochemical reactions and because compounds employed as antifreezes such as glycols and salts are, at least in some cases, toxic and/or capable of damaging the environment.
Furthermore, merely employing a selected magnitude drop in the level of the detecting liquid--which is all that the patents disclose--may allow serious leaks to go undetected. Leaks into a buried system from the surrounding environs, for example, may actually cause the level of the leak detecting fluid in the system to rise.